1. Field of the Invention
The invention relates generally to the semiconductor power devices. More particularly, this invention relates to configurations and methods for manufacturing of new and improved edge terminations for semiconductor power devices to generate electrical field as saddle junctions to reduce the areas occupied by the termination areas while maintaining high breakdown voltages.
2. Description of the Prior Art
Conventional manufacturing technologies and device configuration to further increase the breakdown voltage of a semiconductor power device at the terminal areas are still confronted with difficulties that the termination areas occupied large areas. The difficulties are becoming even more severe and limiting when the semiconductor power devices are now manufactured with ever reduced size. A general survey discovers that for integrate circuit (IC) chips of larger size; the edge termination takes up about 20% of the total area.
However, as size of the chip becomes smaller and smaller, the percentage of the edge termination gradually increases and may take up to about 50% of the total area in order to maintain a high breakdown voltage. The termination area is a “non-active” area of a transistor because it is not useful for current conduction. Even though large areas taken up by the edge termination leads to wastes of valuable active cell areas, there are still significant difficulties in reducing the edge termination area while maintaining a high breakdown voltage.
FIGS. 1A and 1B are cross sectional view of an ideal PN junction with parallel plane breakdown voltage, and an un-terminated PN junction at the edge of a vertical power device to explain the need of improvements for edge termination. FIG. 1A shows an ideal PN junction between a P+ layer and an N− layer on top of an N+ substrate. Due to the doping difference the N− depletion layer 103 is much wider than the P+ depletion layer 101. Ignoring the edge effect the electric field within the depletion region is evenly distributed with parallel plane avalanche breakdown. This is the theoretical limit of Breakdown voltage that can be reached for the given doping levels and thickness of the lightly doped drift region when ignoring the edge effect. However, the actual breakdown voltage of a reversed bias junction can be severely degraded at the edge where the formation of P+ layer by implantation terminates, due to field crowding from the formation of a cylindrical junction, as shown in FIG. 1B. It can be observed that the shape of the depletion region is convex, and that leads to the electric field crowding near the junction.
In order to mitigate this problem, several edge terminations have been proposed, and are widely used in the industry. Some of these include the floating guard rings 90 of FIG. 1C and the Electric Field plate 92 of FIG. 1D. These techniques improve the Breakdown voltage by spreading the depletion region at the surface and thus lowering the electric field. However, these methods typically require large area for implementation, and cause an increase in the die size of the device. In addition, these techniques are prone to surface charges coming from the passivation films and/or the packaging mold compound.
Therefore, a need still exists in the art of power semiconductor device design and manufacture to provide new and improved configurations of the edge termination such that the above discussed problems and limitations can be resolved.